The landing guidance functions provide assistance to aircraft pilots in landing under reduced visibility conditions making visual references insufficient to perform a traditional manual landing. They in particular comprise an automatic landing function that automatically controls the aircraft based on the predetermined guidance instructions and/or a display function on a crosshairs indicating an instruction to the pilot allowing him to pilot his aircraft manually with few outside visual references.
Only two types of sensors deliver the ground speed (Vsol) of the aircraft during flight, in particular during the landing guidance phase: the inertial units and the satellite navigation receivers.
The algorithms that calculate the guidance instructions generally use that speed of the aircraft relative to the ground, which makes the use of that datum critical. In fact, the loss of that speed Vsol or an undetected error in that speed Vsol may lead to guiding the aircraft outside the area where it must land, which can be fatal for the aircraft. It is therefore necessary to secure that ground speed datum by confirming its accuracy or the presence of an error.
It is known to secure the ground speed by comparing the measurements Vsol provided by at least two independent inertial units onboard the aircraft. This solution is nevertheless expensive, because it requires the installation of at least two inertial units.
As a reminder, an inertial unit, also called IRS (Inertial Reference System), generally comprises three rate gyros measuring the three components of the angular speed vector (roll, pitch and yaw speeds) and three accelerometers measuring the three acceleration components. The IRS is suitable for a precise calculation, by measurement integration, of the attitude angles (roll, pitch and heading), the ground speed and vertical speed components, and the position of the aircraft.
One alternative to using two IRS's to calculate and secure the speed Vsol is to use a satellite navigation receiver, also called GNSS (Global Navigation Satellite System), in place of an IRS or a set of IRS's. The issue then arises of the integrity and availability of the satellite data, in particular at low radio altitude. Furthermore, the redundancy of the satellites onboard the aircraft does not make it possible to offset the problem of unavailability of the satellite signals themselves.
There is therefore a need to secure the measurement of an aircraft's ground speed taken into account during landing guidance, reliably and while limiting the necessary investments.